Archives for April 2011

I have been storm chasing since 2004 and have now seen 43 Tornadoes with 2010 being the best season yet with 22 Tornadoes in 6 weeks.

On the 30th May 2010 we headed up towards Oklahoma City. That night an intense hailstorm erupted and moved towards our location at the Hotel, this Storm went on for a good 5 hours and kept some of us awake most of the night with the vivid lightning and loud thunder.

We awoke to a Slight risk for thunderstorms in SE Colorado so I gathered the troops and went west from Oklahoma city towards the Oklahoma panhandle. Models were showing juts a 2% risk for a tornado today and when this happens it goes one of two ways, nothing much of note or that rare day in the year when the 2% is easily succeeded. Luckily it was the latter today and one of the most photogenic tornadoes in the last 6 years would be observed by us.

At around 5pm a Supercell that had been ongoing for a few hours started to drop a few funnel clouds and weak tornadoes around the Springfield area of SE Colorado (the same place we had seen another tornado just 6 days earlier). We gassed the car up and headed even further west hoping the show would not be over by the time we got there. Upon approaching Boise City in the OK Panhandle we finally got a view of the base of the storm to our North West. It had a beautiful Classic Supercell look to it and had a Wall cloud hanging in the SW Quadrant. I took a north farm road blasting through a smaller storm a few miles up that road that contained some small pea sized hail and stopped back in the dry about 5 miles SE Of the almost stationery Supercell.

We all got out of the car to take some pictures of the structure of the storm when a powerful funnel cloud appeared almost at that instant (it was almost as if it was waiting for us to park up and get the cameras all set-up on the tripod).

We were then treated to a Wizard of Oz type twister which elegantly snaked it's way to the ground from the rotating wall cloud, the storm was still moving towards us from the North West so we had lots of time to observe this beautiful tornado in bathed sunlight and temperatures of 80f, a warm inflow breeze from the South East prevailed all the time. The tornado was now morphing into a large cone tornado and was on the ground for a full 17 minutes out of the rain.

After that time the wet RFD (Rear Flank Downdraught) made the tornado go back into the rain and we briefly lost sight of it, but after about 5 minutes the 2nd tornado would appear clearing the rain out and a lot closer to us, this tornado only lasted about 3 minutes, the last tornado appeared about 10 minutes after and only lasted for about 2 minutes.

The storm was by this time almost at our location and losing it's power as it moved more southerly now instead of south-easterly (clearly the boundary it had left as it moved direction had a marked effect on it's lifespan) Numerous other storms were erupting now as darkness fell and at one point on the drive back to our overnight location in Liberal (Kansas) we were surrounded by 4 Supercells which were giving us a 360 degree view of lightning. We punched through a few of the storms cores on the drive back and the new chasers who had just witnessed that as a first chase day thought every day was going to be like that, if only they could.........

Really hoping 2011 will be as active as last year but once again seeing Tornadoes over open prairie as opposed to built up areas is desired. We set off on 28th April and head to central US. We hope to keep you posted right here on the BBC 23 Degrees blog and on netweather.tv.

The right conditions for tornadoes can occur any time of the year in any location. With no start and end date for 'tornado season' it's interesting to explore why there will be a peak in occurrences over the up coming months.

As the continents warm up due to the tilting Earth, the air close to the ground is warmed up too. Spring is a transitional period for the climate, and there are more chances of cooler air meeting with warmer air. This leads to instability in the atmosphere and upwelling convection. Where this convection is strong, storms can develop and where the convection is really really strong this can spawn a tornado.

The unique positioning of the Great plains in US - areas of which are dubbed tornado alley - means that it will see a peak in tornado occurrences through spring and summer as a result of it's bordering air masses. It's a question of geography. Although tornadoes have been reported in mountainous areas, the relatively flat land in the Great Plains creates the perfect conditions for a severe storm. The cold dry polar air from the Rocky mountains of Canada meets with warmer moist winds from the Gulf of Mexico. A large number of tornadoes form when these two air masses meet, along a phenomenon known as a "dryline."

For most people in the UK, the raw power of a tornado is hard to imagine.
But tornado season has started in the US, and reports suggest that 62 tornadoes have ripped through North Carolina alone in the last few days.

During the next few weeks, thousands of scientists and storm-chasers will head to "Tornado Alley" in the hope of studying and witnessing a giant twister. Where do these massive whirlpools in the sky come from?'

First of all, you need an energy source. In the case of a tornado the energy comes from water vapour, which is a bit like an aqueous battery. When liquid water gets hot enough to evaporate into a gas, it doesn't just need to be the right temperature, it also needs a little extra kick of energy (known as latent heat) to convert from a liquid to a gas. When the water molecules are floating around as vapour, they carry this extra energy with them and when they turn back into a liquid they give that energy back to their surroundings. When water from the ocean evaporates and blows away, it carries this stored energy with it. Harmless though water vapour sounds, it's fantastically effective storm fuel.

The storm starts when warm moist air starts to rise, and since this air is full of water vapour it's carrying lots of energy. As it rises, it releases that energy and starts an updraft, an upside-down waterfall in the sky. This stream of air rushes upwards and new air is sucked into the gap that is left behind. Above, condensing water forms a huge cloud.

Tornadoes form in "supercell" storms, which have one really strong updraft that sucks in air from the surroundings at an enormous rate. In your bathtub, water starts to rotate before it's sucked downwards, because it can't all fall down the hole at once. In the same way, air being sucked towards the updraft rotates, but instead of falling downwards in the centre, it rushes upwards. These upward winds can reach speeds of 100 mph. Just like a skater pulling their arms in, the air coming in from the sides rotates faster as it gets closer to the centre. These upside-down waterfalls officially become tornadoes when they cause surface winds faster than 40 mph.

So the warm moist air has fueled a huge and destructive rotating column of air, a tornado. But air is usually invisible. Why can we see twisters?
What we're seeing are water droplets. There is really low pressure in the tornado centre, up to 10% lower than the surrounding air. As air is sucked into this core, the low pressure allows the water in the air to condense much closer to ground level than normal and it's these droplets that we can see. So when you see a funnel cloud from a tornado, you're seeing the shape of the region of low pressure. All the surrounding air is being sucked into that "hole" in the atmosphere.

If you want to see a tornado, you'll need patience and luck. Less than 20% of all supercell storms form tornadoes and scientists are still not sure what makes some storms generate twisters while others don't.

You don't have to be in the US to see tornadoes though. According to the Tornado and storm research organization, an average of 33 tornadoes per year are reported in the UK. They're small ones, but they still count.

Thunder sends some people rushing to the window to watch one of nature's most exciting performances, while others just hide under the bed until it's all over. And just imagine what it must have felt like for most of our ancestors, without the shelter of brick or stone houses. It's easy to see how they might have thought that this enormous noise signaled the approach of Thor, a fierce god known for waving a hammer around. So where does all that sound come from?

You probably know that lightning is a flow of electricity between clouds and the ground. But unless you're having a bad day involving water and a toaster, electricity is silent as it flows through the wires in your home. Lightening is different because there's no permanent "wiring" in the atmosphere - the thunderstorm has to make its own. This happens when a small amount of electric charge finds a path to the ground, ionizing the air along a tube just a few centimeters wide. This tube of air is the "wire", a conducting path for the electricity. Until that connection is made, the electrical charge in the cloud just keeps building up and so by the time the "wire" is ready, there's a vast amount of energy poised to pour along it. For about twenty millionths of a second, a 10,000 amp current rushes down towards the ground (the fuses in your house can't cope with anything greater than 13 amps). The air in that tube is heated up to about 30,000 degrees C - five or six times as hot as the surface of the Sun. As it gets hotter, it expands outwards so quickly that it thumps into the air around the lightning strike. This thump is the thunder. You're listening to the sound of cool air being struck by a plasma hammer much hotter than the surface of the sun. No wonder it's so loud!

The sound then races out sideways in a constantly widening circle, a bit like the ripple from dropping a pebble in a pond. Sound in air travels about 340 metres every second, so if you're a mile from the lightning, you hear the thunder start 4.7 seconds after you see the light. But if thunder comes from an event that lasted a tiny fraction of a second, why do we hear a rumble that goes on for many seconds? The answer is that sound travelling to us from higher up the lightning bolt takes longer to reach us than the sound that only has to travel sideways along the ground. You're hearing different parts of the lightning bolt as time goes on, starting from the ground and moving upwards. It's as if you could freeze the lightning bolt in time and then let your glaze sweep along it, from the bottom to the top.

I love thunderstorms, and I think that it's amazing that you can hear plasma hammering on our atmosphere whenever the right kind of storm is happening. But I will be watching from inside, where it can't hammer on me

April is a month always linked with the occurrence of showers - but are showers really more likely in April than any other month of the year? Of course not, although there are reasons why April might be more associated with them.

As far as weather patterns are concerned, the energy received by the Sun plays a pivotal role on weather systems around the world. The differential heating of the Earth's atmosphere ultimately produces jetstreams - narrow but very long bands of strong winds high in the troposphere.

Jetstreams meander north and south through the year and as a rule during the northern hemisphere spring, jetstreams generally drift northwards and away from the British Isles.

As the main drivers of changeable weather, jetstreams often bring spells of continuous heavy rain. However, as the jetstream moves away from the UK, it leaves the country more likely to have weaker bands of rain and more days when showers are likely to develop.

At this time of year the energy from the Sun is intensifying. As a result, ground temperatures begin to respond to the extra solar heating and warm quickly.

Strong surface heating is another factor typical for generating showers. The other required element is colder air aloft. Although warm at the surface in any sunshine, April is a month when the air higher up in the troposphere can be much colder.

On showery days, air temperatures always fall with height in the troposphere. Thanks to the strong heating, air at the surface has much more energy when compared to the colder air above it - and rises. If conditions allow, the air will be able to continue rising and reach the Tropopause at a level of 30,000 feet or more.

As this air rises up through the atmosphere, it cools and condenses into clouds. Once these clouds develop in sub freezing air and are thick enough they produce showers of rain, some of which will be heavy with thunderstorms and hail.

Showery - or convective days are often accompanied by intervals of sunshine as part of the atmosphere displaying 'convective overturning'. This is because not all air can rise at the same time - it's a case of what goes up must come down. So, a shower is where the air is rising, but areas where the air descends will often be clearer or have thinner cloud.

Clouds associated with showery days are Cumulus and sometimes the thunderstorm cloud Cumulonimbus.

During an April shower, temperatures can vary widely. Outside the shower and in the spring sunshine, temperatures can easily reach 20 Celsius. However, a heavy shower with hail can see these temperatures fall rapidly to around 10 Celsius.

This week we asked you to look up and see 'what's above your head?' - you have done just that. The team have been looking through your photos from the BBC 23 Degrees photo pool - and I must admit there are already a great selection of photos. Each week we will be picking a photo for you to discuss.

Our team are on standby waiting for your comments. As always as and when you see it, whether it be a particularly interesting cloud, another weather phenomena or a pivotal moment signifying Earth's journey for 2011, add them to the BBC 23 Degrees photo pool - and who knows it may well find it's way to the blog or create an interesting debate.

As we say goodbye to winter and enjoy the start of spring there's one thing here in the Northern Hemisphere that most of us don't notice, but never the less we should be thankful for. Our winter ends sooner than it does in the southern hemisphere - 4 days shorter in fact. Which is good news for us because we get four less days of colder weather but bad news for people down-under who get four days more.

It's all down to our orbit around the sun. We can't really see it, but it's not a perfect circle, it's an ellipse with the sun slightly offset in the middle. This was first explained by Johannes Kepler in 1609 in his first law of planetary motion.

The part of the orbit when the earth is closest to the sun is called Perihelion and currently it occurs during the Northern Hemisphere winter. At Perihelion the earth is 5 million kilometres closer to the sun than in July when it's at it's furthest.

So why does that have an effect on the length of the winter. Well that's down to Keplers second law. This states that a line joining a planet and the Sun, sweeps out equal areas during equal intervals of time. Ok that's the scientific description of the law but what does that mean when it comes to our journey around the Sun? Put simply the law explains that when the planet is closer to the sun it moves faster round its orbit, and when it is further away it moves slower.

So during Perihelion and the northern hemisphere winter the planet is moving faster making the season shorter. The southern hemisphere winter is four days longer because it occurs at aphelion when the planet is further away from the sun and therefore moving slower.

But there's a flip side to this because that means that the Northern Hemisphere summer is four days longer than the southern hemisphere summer. Which helps explain why July is our planet's warmest month, At that point the Northern continents that are pointing towards the sun get a few days more to bake in the sun and raise the average temperature of the entire globe.

Most of the other planets in our solar system have orbits that are more elliptical than Earth's. The dwarf planet Pluto's orbit is the most eccentric of all and is so lopsided that at its Perihelion it is actually closer to the sun than Neptune.

Mars's orbit is not as elliptical as Pluto but a lot more than Earth's. At Perihelion Mars is 43.5 million kilometres closer to the sun than at Aphelion and receives 40 percent more sunlight, with an abundant rise in temperatures of around15-20 degrees. The larger degree of eccentricity in the orbit also affects the lengths of the seasons because the planet moves slower at Aphelion, as it is further away from the sun. As a result the Northern Hemisphere summer is not four days longer than the northern winter, but 25 days longer. This has a big impact on the Red Planet's seasons. Northern summers are long and cool while the winters are short and mild. Conversely for the southern hemisphere, summer is short and relatively hot while winter is long and cold.

So we should be quite glad that our orbit is only slightly elliptical - we barely notice our shorter winter, but if we were on Mars we certainly would.

On day 95 of our orbit around the Sun I have only one thing on my mind and that is clouds. Cumulus in particular. Yes it's the most recognisable cloud type but what better represents fairer weather than fluffy cotton like clouds?

Clouds are formed on the lowest major level of the Earth's atmosphere but to us down here clouds appear to be the cushion between us and space, it's the first visible thing we see when we look up and just knowing that little bits of cloud fall down on us makes me even more fascinated by clouds. But what is it about cumulus clouds that make me stare into the sky and snap away with my camera?

Cumulus clouds have a noticeable vertical development and defined edges. In Latin cumulus means pile or heap - just picture a heap of clouds, what a great photo. But the most intense cumulus clouds can be associated with severe weather phenomena such as hail and tornadoes and that is very significant, especially for storm chasers.

If you are up to the challenge - we encourage you to look up and see what type of cloud you can spot using the Royal Meteorological Society photos and description below as a guide. From cirrus clouds to stratus clouds, nimbostratus to cumulonimbus clouds we want to see them all - either add your photos to our Flickr pool or send it to us by email and let's see what type of clouds are out there - the best photos will be published on the blog each week - we will invite the photographers to tell us more about the story behind their photos.

On your journey you may even come across a new classification of cloud like the Undulatus asperatus a cloud put forward for a separate cloud classification in 2009 by Gavin Pretor-Pinney founder of the Cloud Appreciation Society.

It's quite murky now - looking outside my window - but with spring comes a variation of atmospheric conditions and the Sun may well come out tomorrow - I promise not to break out into a show tune. Cumulus is still my favourite, what's yours? Feel free to let us know of any other type of clouds not included below:

In my previous post I mentioned that clouds of snow crystals are above us all year round, thousands of metres above us in the atmosphere. In this post I want to show you how we can continuously observe these ice crystal clouds using radar. Radar works by transmitting a short burst of radio waves (a 'pulse'), then listening to see if part of that pulse is reflected back. By timing how long it takes for the reflections to come back to the radar, we can work out how far away the reflecting object is. Most people are probably more familiar with radar being used to detect aircraft. But just as aircraft reflect radio waves, so too do snowflakes - the only difference is that the reflections are a lot weaker.

So if we want to monitor snow crystals above us, we need a sensitive radar. Fortunately the UK has one of the best facilities for this in the world: the Chilbolton Observatory in Hampshire. Founded in the 1960s for radio astronomy work, since then a number of different radars, remote sensing and meteorological instruments have been installed there making it perfect for cloud and precipitation research. Crucially, a significant number of instruments are now operated 24 hours a day, 365 days a year, letting us continuously monitor the properties of clouds. Here's an example of a cirrus cloud:

(c) Chris Westbrook

On the left here you can see the signal we measured using our 'Copernicus' cloud radar - the colours are the reflections from the snow crystals. The warmer the colours, the bigger the crystals are. On the right you can see a photograph taken by an automated sky camera - this is what the same cloud looks like to the naked eye. In spite of how tenuous the cloud looks in the photograph, the radar demonstrates this is a big mass of ice: the cloud is 3km (just under 2 miles) deep. The streaky character of the cloud is very obvious in both images - this is caused as ice particles are formed initially in narrow regions where the air is rising near the top of the cloud. The crystals then grow, and fall, and as they do so they form these streaks. The streaks are curved because the wind is strong up at 8000 metres, and this blows them off-course as they fall.

We can measure how fast the crystals fall by making use of the 'Doppler effect' where the reflected pulse is shifted in frequency (this is the same as when a police car siren is high pitched coming towards you and lower pitched as it goes away from you). In this case the crystals were falling at about 1m/s, so it would have taken them a bit less than an hour to fall from top to bottom of the cloud. You'll see when they get to 6000m the crystals seem to disappear. This is because the air beneath is too dry, and the crystals evaporate before they get anywhere near the ground. This saga goes on all the time above our heads when there is cirrus around, it is incredible. Cirrus clouds actually warm the planet slightly (a greenhouse effect), and because they are so common across the globe, it is very important to understand how they work so we can get them right in climate models.

The second kind of cloud I'd like to show you is another common one - but one which we are now discovering is much more common than was previously thought. These clouds contain 'supercooled' water droplets. Amazingly these water droplets do not freeze, even at temperatures far below 0 degrees C. Because the droplets themselves are so minute (1/10th the width of a human hair) normal radar can barely detect them. But if we exploit a different kind of radar, which uses infrared light rather than radio waves, we can see exactly where these droplets are. It's called 'lidar'. Here's an example from back in December last year when things were very chilly and we were getting some snow:

(c) Chris Westbrook

The top picture is the lidar - you can see that red stripe at the top of the cloud - that's the supercooled water - it's really reflective to infrared light. The green stuff faling underneath is snow crystals, formed as some of the droplets freeze. The bottom picture shows the radar image - this just detects the snow crystals. The top of this cloud is -13 degrees C, and it's amazing that the liquid can be this cold without all freezing. Actually our work has shown that even as cold as -25C, around half the snow crystal clouds in the atmosphere have supercooled water at the top of them! Exactly how this water persists at top of these cold clouds is still not clear: even though the droplets do not all freeze, the presence of other ice crystals around them should in theory make them evaporate - but they don't. This is important, because the droplets reflect a lot of sunlight (just the same as they do to our lidar pulse), cooling the planet.

That's a small sample of some of the things we can observe at Chilbolton. If you want to see what's above your head right now, check out the realtime images.

About this blog

Orbit: Earth's Extraordinary Journey explores the relationship between the Earth's orbit and the weather. Previously '23 Degrees' (working title); on this blog the weather community were invited to discuss their experiences of severe weather as and when events developed and share their iwitness footage throughout 2011. The audience were provided with an insight to the making of the series and exclusive behind the scenes footage. Follow us on Twitter.

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